How are hormonal effects on sexual behavior mediated at the level of individual neurons or neural circuits? As logical steps toward answering this question, the proposed research aims to achieve: 1) a description of the neural circuitry controlling female sexual receptivity in the golden hamster, and 2) analysis of the functions of particular parts of the circuitry. Recent studies suggest that estrogen target neurons in the region of the ventromedial nucleus of the hypothalamus (VMN) are critically important for sexual receptivity. We have found that VMN lesions, or knife cuts lateral to the VMN, can eliminate sexual receptivity in the hamster. These data suggest that critical VMN connections pass through the region of the supraoptic commissures (SOC). These SOC connections may include both VMN afferents and efferents, and the proposed experiments will examine the possible importance of particular afferent and efferent connections of the VMN. For example, one of the newly discovered VMN afferent cell groups is located in the nucleus of the lateral lemniscus, and axons from these cells may reach the VMN via the SOC. The idea that these neurons may play an important role in female reproductive behavior by relaying somatosensory or auditory information to the VMN will be tested. The techniques to be used will include detailed measures of female mating behavior following stereotaxic placement of microsurgical cuts and chemical lesions, the use of horseradish peroxidase histochemistry for neuroanatomical studies, and electrophysiological recording of neural responses to sensory stimuli. These experiments should begin to tell us how hormone-sensitive neurons in the hypothalamus can influence other brain regions to produce changes in sex behavior, and also how VMN afferent cell groups might participate in this control.